And here is where the money comes in... admit to the fact that auctioning items for real money will happen anyway, and design around that fact, integrating auctioning features into the software and taking cash commissions on the sales of characters, houses, and items. Make it so that secure trades and bartering can only happen in town at x location -- every other exchange is at your own risk for unidentified goods.
Facilitate a complex economy where players are essentially passing money back and forth between each other, kind of like a game of poker, but with no gambling involved. Some people would even be able to make serious money by playing the game.
Of course, one could argue that if you spend real money for a dragon-slaying longsword and magical fire-resistant armor, and then you use a random number generator to determine whether you kill a dragon and can sell its treasure for real money, you *ARE* gambling.
Either way, it could be just as addictive...